The present invention relates to an electronic counting balance, and more particularly to a computerized electronic balance contrived to derive the number of items from a measured weight of the items through an arithmetic operation dividing the measured weight by an estimated unit weight, that is, an estimated average weight per one of the weighed items.
It goes without saying that the precision of this type electronic counting balance depends essentially on the accuracy of the estimated unit weight. Accordingly, there have been contrived many processes of estimating the unit weight. These are roughly classified into two methods.
According to one of the two methods, first a predetermined known number of sample items are weighed to estimate an average weight per one of the whole items whose number is to be counted. The average weight is otained by dividing the measured weight by the above predetermined known number. The thus obtained average weight is employed as the basic unit weight to calculate the number of the whole items. Namely, after the unit weight is obtained, an unknown number of remaining items are additionally loaded on the counting balance to measure a total weight of the whole items, the total weight is divided by the unit weight to obtain the total number of the whole items.
According to this method, it is desirable to choose the above predetermined number to be as large as possible, since the reliability of the unit weight, and therefore, the precision of the calculated number of items is increased with an increase in the number of sampled items. Therefore, it is an important disadvantage of this method that an initial non-instrumental operation of counting a large number of sampled items is not only troublesome but also apt to be accompanied by miscounting.
According to the other of the two methods, after a known number of items are weighed to estimate a first unit weight in the same manner as that used to obtain the unit weight in the previous method mentioned above, a first new total weight is measured with a first unknown number of items replenished, and then the first new total weight is divided by the first unit weight to obtain the present total number of items. Next, the first unit weight is replaced with a second unit weight, which can be estimated by dividing the above new total weight by the present total number of items. Then, a second new total weight is measured with a second unknown number of items replenished, and the second new total weight is divided by the second unit weight to obtain the total number of items at this stage. The similar item counting processes by means of replenishing items successively and renewing unit weights are continued repeatedly until a predetermined whole number of items are counted up.
In the above method of counting items, the reliability of each estimated unit weight essentially depends on each number of replenished items. The maximum allowable number of items to be replenished at each stage of item replenisher is necessarily determined with the weight coefficient-of-varation C.sub.v of items taken into consideration. In Japanese Laid-open Patent Application No. 60-31023 is shown a means for assuring the estimated unit weight of a definite accuracy by determining a maximum allowable replenishing number of items at a probability of 0.997, with a normal distribution supposed. However, this means still has a disadvantage that, if the weight coefficient-of-variation C.sub.v exceeds 2 or 3%, the maximum allowable replenishing number mentioned above can not be so large that the resultant efficiency of successive item counting operations decreases considerably.